1. Field of the Invention
The present invention relates to a method and apparatus for treating wastes by gasification, and more particularly to a method and apparatus for treating wastes by two-stage gasification to recover metals or ash content in the wastes in such a state that they can be recycled and gases mainly composed of carbon monoxide (CO) and hydrogen gas (H.sub.2) for use as synthesis gas for hydrogen gas or ammonia (NH.sub.3).
2. Description of the Prior Art
Ammonia is a mass-produced basic material for producing nitric acid, fertilizers including ammonium nitrate, ammonium sulfate and urea, acrylonitrile, caprolactam or the like. Ammonia is catalytically synthesized from nitrogen gas (N.sub.2) and hydrogen gas (H.sub.2) under a high pressure. Hydrogen gas has been produced by either steam reforming of natural gas or naphtha, or partial combustion, i.e., gasification of hydrocarbon such as crude oil, heavy oil, bottom oil, coal, pitch or petroleum coke.
Since most of materials for producing hydrogen gas are dependent on importation from abroad, ammonia-derived chemical products have lost their competitiveness on the world market after recent oil crises. Therefore, there has been a strong desire for obtaining materials which are inexpensive and available domestically.
It has heretofore been customary to treat organic wastes such as municipal wastes, plastic wastes including fiber-reinforced plastics (FRP), biomass wastes, and automobile wastes by incineration to reduce their volume for reclaiming, or to discard them in an untreated state to landfill sites.
Therefore, only a small quantity of resources has been recovered from the organic wastes for the purpose of recycling, irrespective of direct or indirect utilization.
On the other hand, the incineration of organic wastes has been disadvantageous for the following reasons:
A stoker furnace or a fluidized-bed furnace has heretofore been used for the incineration of organic wastes. However, this incineration has been problematic with respect to environmental conservation, or recycling of resources or energy. To be more specific, large quantities of exhaust gas are discharged because of a high air ratio, and toxic dioxins are contained in the exhaust gas. Further, metals which are discharged from the furnace are not suitable for recycling because they are severely oxidized, and landfill sites become more scarce year by year. Recently, the number of waste treatment facilities which incorporate ash-melting equipment is increasing, however, a problem is encountered in the increase of construction costs and/or operating costs of the waste treatment facilities. Further, recently, there has been developing a tendency to utilize energy of the wastes more efficiently.
Dumping the wastes in an untreated state on reclaimed land has become more difficult because of scarcity of landfill sites, and has not been allowable from the viewpoint of environmental conservation. Therefore, the problem of disposing of harmful wastes such as shredder dust from scrapped cars by landfill is getting worse and worse.
Further, in the case where a large quantity of steam is used with oxygen gas (O.sub.2) as a gasifying agent in the fluidized-bed reactor, operating cost increases. Even if air which is easily available is used as the gasifying agent, there is a limit to the amount of air because of a limitation on the amount of nitrogen that can be used for synthesis of ammonia.